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I don't find horticulture-grade pumice available locally. So, I bought some scoria and sifted it. I got three grain sizes, with two shown here: scoria grain size

The third is whatever passed through the finest sieve. It is the finest, still coarser than regular sand but full of dust. I have kept it until I find what to do with it.

My question - Which to use for the following plants: Haworthia, Gasteria, Aloe, Echeveria and its allied species. Is the finer grain of any use?

My climate - Csa according to the köppen climate classification: hot Mediterranean.

Edit: Climatic data for Tel-Aviv in this link. No rainfall in summer (but very humid), high rainfall in winter.

Do I need to add water-retaining mediums, such as peat or vermiculite? I grow my collection items in unglazed pots. Rooted cuttings from trimming are grown in plastic pots.

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You don't mention where you are, but peat and vermiculite are two of the least desirable water retention materials to use, in my opinion. Calcined clay, or oil absorbent, pumice, etc are much preferred. The oil absorbent is found in autoparts stores. Coconut coir is superior to peat in that it is much easier to rewet once it dries and it doesn't break down as quickly. Are there any hydroponic stores near you? They usually have this sort of material.

  • Just added climate & location info. – Christmas Snow Apr 29 at 13:24
  • Hydroponic clay is available. Tell me if this is the correct grain size: university.upstartfarmers.com/blog/… – Christmas Snow Apr 29 at 17:22
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    That stuff is pretty big. I usually find material that is a little bigger than rice grains, or about the size of orzo. The page also mentions that hydroton doesn't absorb water very well. Maybe you can find oil absorbent or cat litter. The type you want has been baked so it absorbs but doesn't dissolve. There has to be a solution of some sort! – Tim Nevins Apr 29 at 21:05

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